1. Field of the Invention
The present invention relates to a dielectric filter and a dielectric duplexer which comprise a dielectric block having holes with conductors formed on the surfaces of the holes (hereinafter referred to as inner-conductor holes) and an outer conductor on the outer surface of the dielectric block, and to a communication device using the dielectric filter and the dielectric duplexer.
2. Description of the Related Art
A known dielectric filter having a substantially rectangular parallelepiped dielectric block will now be described with reference to FIGS. 7A and 7B.
FIG. 7A is a perspective view of the dielectric filter, and FIG. 7B is a leftside elevational view of the same.
In FIGS. 7A and 7B, the dielectric filter includes a dielectric block 1, inner-conductor holes 2a, 2b, 2c, and 2d, inner conductors 3a, 3b, 3c, and 3d, an outer conductor 4, input and output (I/O) electrodes 5a and 5b, and dielectric resonators 6a, 6b, 6c, and 6d. 
Inside the dielectric block 1, the inner-conductor holes 2a to 2d, which have the inner conductors 3a to 3c on the surfaces thereof, form resonators. On the outer surface of the dielectric block 1, the outer conductor 4 forms a ground electrode. In this manner, the inner-conductor holes 2a to 2d and the outer conductor 4 form the dielectric resonators 6a to 6d. On the outer surface of the dielectric block 1, the I/O electrode 5a is formed separate from the outer conductor 4 and is coupled with the dielectric resonator 6a (hereinafter referred to as the resonator), and the I/O electrode 5b is formed separate from the outer conductor 4 and is coupled with the resonator 6d, whereby the dielectric filter is formed.
However, dielectric filters using the known dielectric blocks have the problems described below.
Generally, bandpass filters must obtain a predetermined attenuation far outside the pass band. To this end, an attenuation pole is provided in the attenuation region of the bandpass filters.
In the known dielectric filter, the I/O electrode 5a is coupled with the resonator 6a and the I/O electrode 5b is coupled with the resonator 6d. Since the amount of coupling between the resonator 6a and the resonator 6b, and between the resonator 6c and the resonator 6d is small, the resonators are not sufficiently capacitively coupled. Therefore, an effective attenuation pole is not obtained. To obtain a sufficient amount of coupling, a capacitor must be provided between the resonator 6a and the resonator 6b, and between the resonator 6c and the resonator 6d. Accordingly, the manufacturing costs increase due to the increase in the number of parts, and the reliability deteriorates because of the increase in the number of connections.
In Japanese Unexamined Patent Application Publication No. 5-145302, a dielectric filter in which an attenuation pole is provided in the vicinity of the pass band thereof is disclosed.
The dielectric filter comprises a dielectric block having a plurality of dielectric resonators. The dielectric block has I/O electrodes on the outer surface thereof, which are coupled with the plurality of dielectric resonators. According to this configuration, since the amount of coupling between the plurality of resonators and between the plurality of resonators and the I/O electrodes becomes large, the attenuation region is provided with a pole, making the attenuation curve steeper by forming an attenuation pole near the pass band. Thus, a bandpass filter capable of sufficiently attenuating unwanted signals is provided.
In the above-described dielectric filter, however, the size of the I/O electrodes may vary according to manufacturing tolerances. In such a case, the amount of coupling between the second resonator and the I/O electrode varies, rendering the attenuation characteristics unstable.
Also, when the width of the I/O electrodes (the dimension of the I/O electrodes parallel to the axes of the inner-conductor holes) is large, the Qo factor of the resonators deteriorates, and the insertion loss increases. Further, since the width of the I/O electrodes is fixed, a dielectric filter formed in a certain shape exhibits only one kind of attenuation characteristic. In other words, the dielectric filter is not capable of exhibiting a plurality of attenuation characteristics for achieving desired characteristics.
Accordingly, the present invention provides a dielectric filter and a dielectric duplexer, which exhibit suitable attenuation characteristics and which readily obtain desired characteristics, and provides a communication device using the dielectric filter and the dielectric duplexer.
To this end, according to an aspect of the present invention, there is provided a dielectric filter comprising a substantially rectangular parallelepiped dielectric block, a plurality of inner-conductor holes extending from a first main surface to an opposing second main surface of the dielectric block, an inner conductor provided on the surface of each of the inner-conductor holes, an outer conductor formed on outer surfaces of the dielectric block, and I/O electrodes extending from side-surfaces to an undersurface of the dielectric block. The side-surfaces are orthogonal to the direction in which the inner-conductor holes are arrayed in the dielectric block, and the undersurface is a mounting surface for facing a mounting board. The I/O electrodes are separated from the outer conductor. Further, at least one of the I/O electrodes extends across portions of the undersurface of the dielectric block adjacent to at least two of the inner-conductor holes when viewed from the undersurface. The dielectric filter obtained according to this configuration exhibits stable and suitable attenuation characteristics.
In one form of the invention, the width of the one of the I/O electrodes is different between a first region corresponding to the first inner-conductor hole and a second region corresponding to the second inner-conductor hole when viewed from the undersurface. According to this configuration, a dielectric filter is obtained, which readily obtains desired attenuation characteristics.
The width at the first region may be larger than the width at the second region. According to this configuration, a dielectric filter is obtained, which readily obtains desired attenuation characteristics.
The width at the first region may be smaller than the width at the second region. According to this configuration as well, a dielectric filter is obtained, which readily obtains desired attenuation characteristics.
According to another aspect of the present invention, there is provided a dielectric duplexer comprising the dielectric filter. According to this configuration, the dielectric duplexer exhibits desired characteristics, and stable and suitable attenuation characteristics are achieved.
According to another aspect of the present invention, there is provided a communication device comprising the dielectric filter. According to this configuration, the communication device exhibits suitable communication characteristics.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.